1. Field of the Invention
The present invention relates to digital radiation imaging. In particular, the invention relates to a radiation device including a multi-functional radiation, e.g., x/gamma ray, identifying and processing application specific integrated circuit (ASIC) for single photon processing imaging with CMOS and solid state pixel detector technology.
2. Description of Related Art
In digital radiation imaging, the spatially varying intensity of a radiation field of interest is recorded by discrete pixels of an image receiving device.
In conventional digital radiography, the intensity is measured as the total energy deposited by the radiation in each pixel of the image receiver.
In other imaging applications, such as in nuclear medicine or back scatter imaging, the number of radiation quanta detected or counted defines the image signal instead of the total integrated energy.
The past years have seen a growing interest in this method of photon counting imaging, including in digital radiography due to the fact that ideally both the image signal to noise ratio (SNR) and the contrast can be improved if the signal photons are given equal weight regardless of their energy. This, as well as noise reduction by discrimination of scattered photons falling below a certain energy, are possible in photon counting imaging.
While integration imaging can be realised with rather simple pixel level electronics, photon counting imaging demands a more complicated preamplifier and usually on-pixel comparator-counter circuitry.
More recently ideas of advanced radiation imaging methods have been introduced. These ideas, such as high resolution position sensitive X-ray fluorescence spectroscopy (XRF), energy selective X-ray imaging or 3D position recording of gamma rays for electronic collimation, require even more sophisticated photon processing than discrimination and counting. Advanced photon processing may include, in addition to counting and discrimination, analog signal amplitude recording and readout, on-pixel analog to digital (A/D) conversion and digital amplitude readout, signal timing deduction, sparse readout, dual energy thresholding, etc.
As a method of imaging, photon counting has been most widely utilized in gamma cameras in nuclear medicine. The limitations of the conventional scintillator-photo multiplier gamma cameras in both spatial and energy resolution have motivated the development of photon counting solid state semiconductor pixel detectors. Especially CdTe/CdZnTe small field of view or so called mini gamma cameras have been introduced first based on discrete detector crystals and later on monolithic detectors flip chip bonded to signal readout ASICs to form a hybrid pixel detector. Photon counting semiconductor hybrid pixel detectors have been developed also for X-ray imaging.
Several pixel Application Specific Integrated Circuits intended for photon counting imaging in connection with semiconductor detectors have been designed and manufactured.
In the late eighties M. Campbell, E. H. M. Heijne et al. introduced the idea of using hybrid pixel detectors in high energy physics experiments (E. H. M. Heijne and P. Jarron, Development of silicon pixel detectors: An Introduction, NIM A 275 (467-471), 1989). The pixel ASICs developed for particle tracking were equipped with pixel level preamplifier, signal shaping and discrimination circuitry and with binary output, i.e., at readout each pixel shows either hit or no hit (E. H. M. Heijne, Semiconductor micropattern pixel detectors: a review of the beginnings, NIM A 465 (1-26), 2001).
The binary output pixel ASICs bump bonded to semiconductor detectors were tested also for imaging of visible photons (C. Da Via et al., Imaging of visible photons using hybrid silicon pixel detectors, NIM A 355 (414-419), 1995), and of X-rays and β-particles (C. Da Via et al., Gallium arsenide pixel detectors for medical imaging, NIM A 395 (148-151), 1997).
In 1998 M. Campbell, E. H. M. Heijne et al. published a description of a photon counting pixel ASIC specifically design for imaging applications. The pixel level circuitry now included a 15 bit counter to store the number of photon events present in each pixel during the image acquisition time. This activity lead to the Medipix collaboration (http://medipix.web.cern.ch/MEDIPIX). The Medipix1 chip is claimed to be the first (full size) photon counting ASIC suitable for X-ray imaging (M. Campbell et al., Readout for a 64×64 pixel matrix with 15-bit single photon counting, IEEE Trans. Nucl. Sci., Vol. 45, Issue 3, part 1 (751-753), 1998; B. Mikulec et al., X-ray imaging using single photon processing with semiconductor pixel detectors, NIM A 511 (282-286), 2003). Its successor, the Medipix2, with a pixel size of 55 μm and with 13 bit on-pixel counters is capable of handling count rates up to 1 MHz/pixel. It offers dual energy thresholding (energy window) and is sensitive to both negative and positive signals (http://medipix.web.cern.ch/MEDIPIX).
In 1997 P. Fischer et al. submitted a report (P. Fischer et al., A counting pixel readout chip for imaging applications, NIM A 405 (53-59), 1998) on a photon counting pixel chip with very similar functionalities to those of the Medipix1 circuit. The same group has also published results on photon counting imaging with pixelated CdTe detectors (P. Fischer et al., A counting CdTe pixel detector for hard X-ray and γ-ray imaging, IEEE Trans. Nucl. Sci., Vol. 48, Issue 6, part 3 (2401-2404), 2001).
Photon counting radiation imaging with a hybrid pixel detector comprising on-pixel counters in one to one correspondence with the detector pixels was disclosed in U.S. Pat. Nos. 6,248,990 and 6,355,923 to J. Pyyhtia and K. Spartiotis. A semiconductor imaging device is disclosed that includes an imaging substrate comprising an image cell array of detector cells, each detector cell corresponding to an individual pixel of the image cell array, and which directly generate charge in response to incident high energy radiation, and a counting substrate containing an array of image cell circuits, each image cell circuit being associated with a respective detector cell, the image cell circuit comprising counting circuitry coupled to the respective detector cell, and configured to count plural radiation hits incident on the respective detector cell, wherein the counting substrate is directly connected to the imaging substrate by bump-bonds. This idea is utilized, e.g., in the Medipix chips.
E. Beuville et al. published in June, 1996, a description on a pixel detector bump bonded to a photon counting ASIC with a preamplifier-shaper-discriminator-counter (3 bit) chain on each pixel (E. Beuville et al., A 2D smart pixel detector for time resolved protein crystallography, IEEE Trans. Nucl. Sci., Vol. 17, No. 3 (1217-1247), June 1996).
C. Ronnqvist et al. presented the idea of X-ray imaging with a hybrid pixel detector in 1995 (C. Ronnqvist et al., Development of a digital X-ray imaging detector, IEEE Nuclear Science Symposium and Medical Imaging Conference, Vol. 3, (1607-1611), October 1995), and published experimental results on a photon counting ASIC in 1996 (C. Ronnqvist et al., A 64-channel pixel readout chip for dynamic X-ray imaging, IEEE Nuclear Science Symposium and Medical Imaging Conference, Vol. 1, (351-355), November 1996). The pixel circuit is similar to the Medipix design and to the S image idea with the only significant difference of the pixel counter being analog. The pixel output is a voltage value proportional to the number of photons above the discrimination level instead of a digital number of counts.
However, it is to be noted that apparently the above mentioned systems have had limited success in applications. The main reason is that although the principle of identifying individual x-ray photons, amplifying them and then incrementing a counter has merits, in reality it is not possible to make a reliable imaging device. The main reason is that calibrating such devices has been very difficult; the information of the energy of the photons is lost once a digital count is used to increment the pixel counter and depending on the application one would want to have different treatment of the photon events. What is needed is a flexible x-ray and gamma ray identifying imaging device that is based on a pixelated electronic architecture with enough functionality on each pixel to allow more than one operations depending on what the user wishes to achieve.